Salinity balance and historical flushing quantified in a high-rainfall catchment (Mount Lofty Ranges, South Australia)

Human-induced landscape salinization, including the effects of dryland salinity, is negatively impacting many catchments in southern Australia. Salinization occurs due to increased recharge and water-table rise following land clearing of deep-rooted native vegetation. In low-lying areas with poor drainage, groundwater-level rise can lead to increased evapotranspiration, mobilization of vadose zone salt, and salt scalding. Alternatively, these same processes of increased recharge and groundwater rise can lead to decreased salinization as accumulated salts are flushed into surface waters. This study in a high-rainfall area of the Mount Lofty Ranges of South Australia documents catchment desalinization. A 28-year record (1989–2016) of flow and salinity in Scott Creek was analysed based on monthly data. Analysis of catchment-scale chloride deposition and export determined that approximately three times more chloride is exported than is input to the catchment from atmospheric sources. Over the time period investigated, salt load exported to surface water decreased by, on average, 6.4 t annually due to catchment freshening. Analysis of monthly salinity balance of a sub-catchment drained by an intermittent stream demonstrates that accumulation of chloride, rather than export, occurs during drought years. In the catchment, as a whole, salts are being flushed via groundwater flow into the permanent stream in all years of the record. Deep groundwater is input to the permanent stream, with mixing of deeper more saline and fresher shallow groundwater. Thus, a complex interaction of landscape hydrologic parameters such as relief, precipitation, chloride deposition and land-use history, determine whether a catchment undergoes salinization or desalinization.